The spine is one of the most delicate portions of the human body, and is subject to failure from a number of causes, one of the most common being failure of a disc positioned between the vertebrae.
Prostheses have been successfully inserted between the vertebrae of the upper spine, but efforts to provide a prosthesis for the discs between the vertebrae of the lower spine have been less successful. Marnay, U.S. Pat. No. 5,314,477 and Salib, U.S. Pat. No. 5,258,031 are typical examples of current efforts to install such prostheses in lower vertebrae. These efforts, however, have not been successful for several reasons. First, existing prostheses have generally disc-shaped upper and lower members, each of which has outer dimensions nearly equal to the outer dimensions of the weight bearing portions of the upper and lower vertebrae, between which the original disc was positioned. To insert such a disc through a patient's back, an incision must be made between adjacent ribs. The disc must then be inserted without interfering with the spinal cord or any of its adjoining nerves. Surgeons have found that an incision large enough to receive a prosthesis cannot be made in the middle or lower back without causing damage to adjacent nerves.
A prosthesis disc might also be positioned between vertebrae of the middle and lower back by inserting it through the abdominal cavity, however, such efforts have also been unsuccessful. Extending along the back wall of the abdominal cavity are parallel arteries carrying blood to and from the lower extremities. Inserting a prosthesis disc through the abdominal cavity requires moving it between the arteries which run adjacent the inner surface of the back. Again, surgeons have been unable to safely insert existing prostheses through the abdomen.
Another problem of existing prostheses lies in their operation. Existing prostheses have upper and lower members, one of which has a generally semispherical portion which is received in a complementarily concave portion of the second member. Such prostheses employ the weight of the upper body to retain the semispherical portion of one member within the concave portion of the other member. When the patient is reclining, however, or when the patient bears his body weight with his arms so as to stretch the spine, existing prostheses are unable to retain their relationship to one another, and the parts may become disoriented. Such disorientation will cause intense pain and render a patient immobile.
It would be desirable, therefore, to provide a prosthesis disc for insertion in the spine of a patient which can be inserted in the middle or lower back and which will not disassemble by the movements of the patient's body.